You can apply for the change in plan from noon on December 27th via the My B-mobile page under “change service” (サービス変更).
However, the cost of upgrading could be significant for some people:
- ¥2,000 fee is charged to change between a 3G and LTE plan (or vice versa in the case of the “keitai denwa” SIM.
- Older SIM cards are incompatible and docomo levies a ¥3,000 fee for a new SIM
The SIM with "docomo" written in red on the right is an LTE-capable SIM card. These SIMs are used for newer voice plans, even if the data in only 3G. The SIM with FOMA written in blue on the left is only capable of 3G and must be exchanged.
If you have the FOMA SIM shown on the left, then you will also have a 3G plan, so you're looking at ¥5,000 to upgrade, which is more than just dumping your old plan and getting the Free Data plan new to start. The question becomes, is keeping your existing phone number worth ¥2,000?